132 INSECT MISCELLANIES. 



Mr Herschel seems inclined to make observations, 

 with regard to the vision of insects, somewhat analo- 

 gous to those of Dr Wollaston in the instance of hear- 

 ing. It may not be improper to give his own state- 

 ment of his views. 'Although,' says he, ' any kind 

 of impulse, or motions, regulated by any law, may be 

 transferred from a molecule in an elastic medium, yet 

 in the undulating theory of light, it is supposed that 

 only such primary impulses as recur according to regu- 

 lar periodical laws at equal intervals of time, and re- 

 peated many times in succession, can affect our organs 

 with the sensation of light. To put in motion the mole- 

 cules of the nerves of our retina with sufficient effica- 

 cy, it is necessary that the almost infinitely minute im- 

 pulse of the adjacent ethereal molecules should be 

 often and regularly repeated, so as to multiply, and, as 

 it were, concentrate their effect. Thus, as a great 

 pendulum may be set in swing by a very minute force 

 oflen applied, at intervals exactly equal to its time of 

 oscillation, or as one elastic solid body can be set in 

 vibration by the vibration of another at a distance, pro- 

 pagated through the air, if in exact unison, even so 

 may we conceive the gross fibres of the nerves of the 

 retina to be thrown into motion by the continual repeti- 

 tion of the ethereal pulses ; and such only will be thus 

 agitated as from their size, shape or elasticity, are sus- 

 ceptible of vibrating in times exactly equal to those at 

 which the impulses are repeated. Thus, it is easy to 

 conceive how the limits of visible colour may be es- 

 tablished ; for if there be no nervous fibres in unison 

 with vibrations, more or less frequent than certain 

 Hmits, such vibrations, though they reach the retina, 

 will produce no sensation. Thus, too, a single im- 

 pulse, or an irregulaily repeated one, produces no 

 light ; and thus, also, may the vibrations excited in 

 the retina continue a sensible time after the exciting 



